How many samples should I use?
You find the right amount of samples by testing what is enough to get a clear image
Samples represent the calculations of how each pixel should look. During one sample a path from a pixel to a light source is calculated. A ray travelling that path is effected by various interactions with surfaces in the scene and this is how the colour of the pixel is determined. The same pixel can be a result of light coming from a lot of directions so the more samples, the more accurate the result, because they each calculate another path checking if it goes to some light source.
So simple scenes with direct lighting need a lot less samples than complex scenes with a lot of indirect light or caustics for the render to have less noise/grain. To determine the correct amount of samples for your scene you should test, how many samples is enough. You can start with fewer samples and make test renders to see if it's enough while increasing the amount. If you do this with the whole image this might take too much time so you can render only regions for testing (ctrl+b in edit mode to define region, ctrl+alt+b to remove region):

It is best to chose the regions that are the darkest lit places that will have the most indirect light and therefore are most likely to contain noise or any areas that you suspect might have more noise. You can also choose progressive rendering and just look at the render while it's going to see how many samples is enough.